When writing a document, the author sometimes wants to include content that not all readers care to see, or which the care to see only at a time of their choosing. One way we do this in a standard print document is a footnote. 1 Footnotes appear at the bottom of the page. In the main text, the footnote is indicated by a superscript.
Other options include endnotes, which appear at the end of the chapter or book, and sidenotes, or which appear to the side of the text in a wide margin.
In a web document, "the bottom of the page" does not mean quite the same thing as it does in a book. The goal of a footnote is to have the note appear close to the text, allowing the reader to glance at them, or not, without interrupting their reading of the main text. A single web page can be quite long 2, which would put a footnote far away from the original text.
A single web page can be as long as a traditional book chapter, or even as a long as a book!